PORT ST. LUCIE, Fla. — Matt Harvey, the Mets’ ace on hold, gripped a baseball in his right hand while gnawing away at a chunk of chewing gum. He slid forward a couple short steps along the right-field grass and launched the ball into a catcher’s mitt 60 feet away. The smack of baseball colliding with leather reverberated around Tradition Field.

In a perfect world, this is the vision of the Mets pitcher fine-tuning his form, just a few Grapefruit League games from an Opening Day start. But reality intrudes.

Harvey was instead completing one of four weekly throwing sessions to strengthen a new elbow ligament after offseason Tommy John surgery. The Mets’ presumptive Opening Day starter, Jonathon Niese, was on a bus to Jupiter, Fla., where he would require 50 pitches to finish a pair of innings against the St. Louis Cardinals.

Niese’s own health has been bothersome this spring. The Mets shipped the left-hander back to New York for an examination of his sore throwing shoulder. The fear was that Niese had aggravated the rotator cuff tear that cost him seven weeks of the 2013 season. The injury turned out to be weakness in a muscle behind Niese’s shoulder blade, but it has nonetheless set him back.

The first pitch of Niese’s Grapefruit League debut was a fastball that clocked 79 mph on the Roger Dean Stadium radar gun. The next fastball registered at 81. Terry Collins, seated alongside the Mets dugout, said he immediately feared a problem with Niese’s arm or shoulder.

The manager was pleased to see Niese’s velocity increase steadily, eventually peaking at 89 mph. Collins pointed to the second inning — when Niese allowed a two-run home run to Kolten Wong and a two-run double to Allen Craig off the wall in center — as the point his starter began to "settle down," even if it took 31 pitches.

"It took a little while for me to get going," said Niese, the Mets’ 2013 Opening Day starter.

Measurables during the spring like velocity — the product of arm conditioning and soundness of a throwing motion — are more important as gauges of a pitcher’s readiness for the regular season than, say, runs or hits allowed. That Niese emerged healthy from the outing qualified as a success in Collins’ estimation.

"He said there’s no discomfort so I’m not worried about it," Collins said.

With ice packs wrapped around his shoulder and elbow — normally used by pitchers to jumpstart recuperation — Niese said his body feels strong. The injury absence, however, set back his throwing mechanics. The moving parts are not yet synchronized.

As a result, "I don’t feel like my pitches are as crisp," Niese said.

To compensate for time lost on the mound, Niese has crammed throwing sessions off flat ground to coordinate his motion. He appeared in a simulated game with teammates last Thursday, also, with the aim to be ready for Opening Day on March 31 against the Washington Nationals.

Collins deemed Tuesday’s outing a step in that direction, but noted it is too soon to place Niese in Harvey’s coveted role as staff ace. Niese will have to demonstrate he can handle the job over three more spring starts, increasing his pitch count first to 65 then eventually to 100.

"We have to make sure we get the right guy to give us the innings we need on Opening Day," Collins said.

If Niese is not ready, other candidates are waiting. Bartolo Colon and Dillon Gee, both right-handers, earned praise from their manager after strong performances Monday.